1. Field of the Invention
The invention relates to the field of semiconductor photodetection, particularly photodetection employing photodiodes and shift registers.
2. Prior Art
The depletion regions of diffused junctions and field induced junctions have been employed for a number of years for the separation of electron/hole pairs which are generated by incident light. Often a doped pn junction (photodiode) in a silicon substrate is employed for this purpose. This doped region is most typically covered with a layer of silicon dioxide, thus the incident light passes through an air/SiO.sub.2 interface and an SiO.sub.2 /Si interface. These interfaces present minimum losses. When induced junctions are employed the incident light is required to travel through a transparent gate member; this introduces two additional interfaces. Moreover, the most commonly employed gate material for this purpose, polycrystalline silicon, absorbs some of this incident radiation, particularly the shorter wavelengths. Thus in many applications the diffused junction is preferred over the induced junction.
Both analog shift registers and digital shift registers are used to read or shift the information sensed by a plurality of junctions to a common terminal or video line. The digital shift register is most often used to sequentially access switches which sequentially connect the junctions to a common video line. With analog shift registers, such as the bucket brigade devices (BBD) and the charged-coupled devices (CCD), charge from a plurality of junctions is simultaneously transfered into the register (in parallel) and then shifted to one end of the register.
Various combinations of the induced and diffused junctions, and analog and digital shift registers are employed in sensing arrays. For example, in U.S. Pat. No. 3,814,846 the combination of digital shift registers and (diffused) photodiodes is described. A plurality of switches are employed to sequentially couple the photodiodes to a common line. One problem inherent in this array is that the non-uniformity of the switches causes a fixed pattern modulation, or noise, which is superimposed on the video output. Moreover, the random noise associated with this array is a function of the output capacitance, and hence, is dependent upon the number of photodiodes in the array. Thus random noise becomes more of a concern with this structure as the number of photodiodes is increased.
In U.S. Pat. No. 3,866,067, the combination of field induced junctions and an analog shift register is disclosed. This structure permits higher density junctions (20 to 30 micron centers), and moreover, has a lower random noise and fixed pattern noise than many other arrays. However, these advantages are offset by the disadvantages that result from process variations. The thicknesses of the layers used to provide the induced junctions, such as the polycrystalline silicone layer, are not uniform. Thus reflectivity as well as absorbtion are not consistent.
It is believed that optimum photodetection may be obtained with a combination of the diffused photodiodes and analog shift registers. This combination is disclosed in U.S. Pat. No. 3,845,295. As shown, the structure includes a row of photodiodes with an analog shift register and transfer means on one side of the photodiodes and the anti-blooming or reset means on the other side of the row of photodiodes. This general architecture is also shown in an article entitled "A charge-Coupled Infrared Imaging Array With Schottky-Barrier Detectors" by Elliott S. Kohn, IEEE Transactions on Electronic Devices, Volume ED-23, No. 2, February 1976.
As will be seen, the photodiode array of the present invention uses the combination of analog shift registers and diffused junctions. However, unlike the prior art, an architecture is described which permits the photodiodes to be interlaced or interdigitated. This allows the fabrication of higher density arrays.